engleski

A single mutation in alfa/beta-fold protein family causes defect in protein processing

A Arg to Cys mutation in the extracellular domain of neuroligin-3 (NL3) was recently found in a twin set with autism and genomic mapping has revealed a correspondence in the locations of chromosomal alterations at the positions of the four neuroligin genes1. The Cys substitution led to specific retention of the mutated NL3 into the endoplasmic reticulum (ER)2. NL3, butyrylcholinesterase (BuChE), and acetylcholinesterase (AChE), as members of the alfa/beta-hydrolase fold family of proteins, share over 30% of amino acid identity and other structural features in their extracellular domains. In particular, Arg451 in NL3 is conserved in the alfa/beta-hydrolase fold family being homologous to Arg386 in BuChE and Arg395 in AChE. A Cys substitution at the homologous Arg in the BuChE was found studying post-succinylcholine apnea in an Australian population3. We have made the homologous mutation in the mouse AChE and BuChE genes and studied the expression, activity and sub-cellular localization of the mutated proteins in HEK293 cells. Consistent with the findings in NL3, we observed that the Arg to Cys mutations resulted in ER retention of the proteins. Our data also indicate that the ER-retained protein was enzymatically active with catalytic constants nearly identical to the wild type enzyme, hence, a global misfolding of protein is not responsible for the retention of the protein. Immunofluorescence experiments showed that the mutated protein co-localized with the ER marker calnexin. Taken together, these data suggest that the ER retention involves the formation of disulfide bonds between ER oxidoreductases and the mutated Cys residue. We demonstrate here that the Arg to Cys mutations results in an identical alterations in the cellular phenotype for the various members of the alfa/beta-hydrolase fold family proteins and we suggest that this specific residue when modified to cysteine influences the processing and secretion of proteins in the alfa/beta-hydrolase fold family. 1. Jamain S. et al., Mutations of the X-linked genes encoding neuroligins NLGN3 and NLGN4 are associated with autism. Nat Genet (2003) 34: 27-29. 2. Comoletti D et al., The R451C-Neuroligin-3 Mutation Associated with Autism Reveals a Defect in Protein Processing. J. Neurosci (2004) 24: 4889-4893. 3. Yen T, et al., Butyrylcholinesterase (BCHE) genotyping for post-succinylcholine apnea in an Australian population. Clin. Chem. 2003 49:1297-308.

alfa/beta hydrolase fold; endoplasmic reticulum; protein processing; defect

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